github-pr-review-mcp

A local MCP server for reviewing GitHub pull requests: it reads PR diffs, reads your coding standards, and posts inline review comments / a full review back to the PR — all driven by whatever MCP client you connect it to (e.g. Claude Desktop or Claude Code).

Scope is intentionally narrow: this server can only read PR/repo data and post PR review comments/reviews. It cannot edit files, push commits, merge, delete, or manage anything else in your GitHub account.

Tools

Related MCP server: code-reviewer-mcp

1. Create a GitHub token

1. GitHub → Settings → Developer settings → Fine-grained personal access tokens → Generate new token.

2. Resource owner: your account (or the org that owns the repos you'll review).

3. Repository access: select the specific repos you want this server to touch.

4. Permissions → Pull requests: Read and write. (Also grant Contents: Read-only if you want github_get_review_standards to fetch a standards file from inside the repo.)

5. Generate and copy the token (starts with github_pat_ or ghp_).

2. Install dependencies

cd github-pr-review-mcp
python3 -m venv .venv
source .venv/bin/activate      # Windows: .venv\Scripts\activate
pip install -r requirements.txt

3. Configure

Edit STANDARDS.md in this folder to reflect the standards you actually want enforced (it ships with a generic starter template — trim/expand it).

Set your token as an environment variable, or wire it into your MCP client config directly (see step 4) — either works, you don't need both.

export GITHUB_TOKEN=github_pat_xxxxxxxxxxxx

4. Register with your MCP client

Claude Desktop — edit claude_desktop_config.json:

{
  "mcpServers": {
    "github-pr-review": {
      "command": "/absolute/path/to/github-pr-review-mcp/.venv/bin/python",
      "args": ["/absolute/path/to/github-pr-review-mcp/server.py"],
      "env": {
        "GITHUB_TOKEN": "github_pat_xxxxxxxxxxxx"
      }
    }
  }
}

Claude Code — from the project directory:

claude mcp add github-pr-review \
  --env GITHUB_TOKEN=github_pat_xxxxxxxxxxxx \
  -- /absolute/path/to/github-pr-review-mcp/.venv/bin/python /absolute/path/to/github-pr-review-mcp/server.py

Restart the client after editing config. Then ask it to run github_whoami to confirm the connection works.

5. Typical review flow

Once connected, a prompt like this drives the whole workflow:

Review PR #42 on ososerp-web against our standards and post comments.

Under the hood the client will typically:

1. github_get_review_standards (local + repo-side)

2. github_get_pull_request (get head_sha)

3. github_get_pull_request_files or github_get_pull_request_diff

4. github_list_review_comments (avoid duplicate feedback)

5. github_submit_pr_review with inline comments and a verdict

Managing context/token usage on large PRs

Tool results stay in context for the rest of the session, so a couple of knobs help keep big PRs from flooding it:

• github_get_pull_request_files truncates each file's patch to max_patch_chars (default 2500) and returns patch_truncated: true when it cut something off. Pass include_patch=false for a filenames-only overview first, then use paths=["src/big_file.ts"] to re-fetch just the file(s) you actually need to inspect closely (optionally with a higher max_patch_chars).

• github_get_pull_request_diff truncates to max_chars (default 8000). For anything beyond a small PR, prefer github_get_pull_request_files instead — it lets you review file-by-file rather than pulling the whole diff at once.

• List-heavy tools return compact (non-pretty-printed) JSON to shave a bit more off every call.

The general pattern for a large PR: call github_get_pull_request_files with include_patch=false first to see what changed, then selectively pull full patches only for the files that look like they need real scrutiny.

6. Automate it: review every PR automatically

Rather than typing a prompt each time, you can trigger a review the moment a PR opens, using a GitHub Actions workflow that runs Claude headlessly.

Note: If your org is on a Claude Team/Enterprise plan, Anthropic also offers a fully-managed Code Review feature (org admin enables it once, no workflow file needed, tune it with a REVIEW.md in the repo). See code review docs. What follows is the self-hosted alternative that reuses this server and your STANDARDS.md directly, works on any plan, and gives you full control over the tools/prompt.

This repo already includes the pieces:

• .github/workflows/pr-review.yml — triggers on PR opened/synchronize/reopened

• mcp-config.json — tells Claude Code how to launch this server in CI

Setup:

1. Copy this whole github-pr-review-mcp/ folder (including .github/workflows/pr-review.yml and mcp-config.json) into the root of the repo you want auto-reviewed. (Or add it as a git submodule if you want it to stay in sync across repos.)

2. In that repo's GitHub settings → Secrets and variables → Actions, add ANTHROPIC_API_KEY (from console.anthropic.com).

3. Push a test PR. Within a minute or two you should see review comments posted by the github-actions[bot] account.

Caveats:

• The workflow uses the default secrets.GITHUB_TOKEN, which only has write access for PRs within the same repo. PRs from forks run with a read-only token and no access to secrets by default (GitHub's security model) — reviewing external contributors' PRs automatically needs a pull_request_target workflow instead, which is more advanced and requires care not to check out/execute untrusted code.

• Each review run costs Claude API tokens (scales with PR size); consider limiting the trigger to opened only (drop synchronize) if you don't need re-review on every push.

Notes on line numbers

GitHub's review-comment API expects line to be a line number that actually appears in the diff hunk for that file (from github_get_pull_request_files' patch field), not an arbitrary line in the full file. If a comment call returns a 422 error, re-check the patch/hunk for that file first.

Troubleshooting

• 401 error: GITHUB_TOKEN missing, expired, or not picked up — confirm it's set in the exact process/config running the server.

• 403 error: token lacks "Pull requests" permission on that repo, or the repo is private and the token/account can't see it.

• 404 error: wrong owner/repo/PR number, or no access to a private repo.

• 422 on posting a comment: the line/path/side combination doesn't correspond to a changed line in the diff.